Such optical pointing devices are already known in the art, International Patent Application No. WO 03/049018 A1, filed in the name of the same Assignee and enclosed herewith by way of reference, describes a motion detection device for detecting relative motion with respect to an illuminated portion of a surface.
As illustrated on FIG. 5, this motion detection device comprises a photodetector array 120 including a plurality of pixels aligned along first and second axes for detecting a light intensity pattern of illuminated portion surface S by means of light source 110. Comparator means 115 are coupled to photodetector array 120 for comparing light intensity between pixels of the photodetector array along both axes and for outputting edge direction data from the detected light intensity pattern. This data is descriptive of light intensity differences between the compared pixels and includes a first edge condition, or positive edge, defined as a condition wherein the light intensity of a first pixel is less than the light intensity of a second pixel and a second edge condition, or negative edge, defined as a condition wherein the light intensity of the first pixel is greater than the light intensity of the second pixel.
The motion detection device further comprises processing means 100 for extracting edge inflection data from the edge direction data. This edge inflection data is descriptive of the succession of positive and negative edges along one of the two axes of the photodetector array and includes a first inflection condition, or peak, defined as the succession, along one axis, of a positive edge followed by a negative edge and a second inflection condition, or null, defined as the succession of a negative edge followed by a positive edge.
Processing means 100 further comprises means for comparing locations of the inflection conditions with previous locations of the inflection conditions, means for determining the direction of motion of the inflection conditions along both axes based on the result of the location comparison and means for computing an overall displacement measurement from the determined direction of motion of the inflection conditions. This motion detection method is called “Peak/Null Motion Detection”.
With “Peak/Null Motion Detection”, the number of inflections “seen” by the photodetector is dependent on the sensor focus. In particular, the inflection count will decrease substantially if the sensor is out of focus (the sensor “sees” less features). This property can therefore be exploited in order to provide “loss-of-focus” information. This can easily be implemented by counting the total number of X inflections and Y inflections and comparing either count (X or Y inflections) with a pre-determined threshold. If this count is less than the predetermined threshold, a “loss-of-focus” condition will be detected and reported as a warning signal to an external controller 200. This “loss-of-focus” information may be used for lift detection of the optical sensor.
A reliable detection of optical pointing device lift condition is essential for low power applications (e.g. wireless mouse). In fact, once the optical pointing device is lifted, the optical system as mentioned before “goes out of focus” and “sees” less motion features. The motion features seen by the sensor become “blurry”, and noise becomes more dominant, the signal-to-noise ratio being strongly decreased. This results in a spurious motion detected by the sensor even if the optical pointing device is not moving (e.g. one side of the optical pointing device is resting on an object taller than the surface in such a manner that the optical pointing device is inclined). This spurious motion will not allow the optical pointing device to enter a so-called “Sleep Mode”, i.e. low consumption mode, since the processing means interpret reported motion from the sensor as an indication that the optical pointing device is being used, and thus would result in a significant power cost.
In the International Patent Application No. WO 03/049018 A1, the lift detection mechanism uses a “hard” or predetermined threshold of number of motion features, for instance the number of edge inflection data, also called hereafter loss-of-focus threshold (LoFth). Once this LoFth threshold is crossed, i.e. number of motion features seen by the sensor is lower than the LoFth threshold, a lift condition is detected.
Since different surfaces present a different average number of motion features to the sensor, for example for a 30×30 pixel array, the average number of motion features is in the range of 90 to 400, therefore no predetermined value can fit in order to obtain both a good sensing motion and a good lift condition detection. If the loss-of-focus threshold is set low, then lift condition will not be detected for surfaces, which exhibit a large amount of motion features to the sensor. In contrast, if the loss-of-focus threshold is set high, then lift condition will be detected even when the mouse is not lifted on surfaces, which exhibit a small amount of motion features to the sensor. This false lift detection will interrupt the normal motion detection of the sensor, which is not user convenient.
That is why the solution presented in the International Patent Application No. WO 03/049018 A1 is either not reliable enough for detecting a lift condition if the LoFth value is set low or not enough sensitive to motion if the LoFth is set high and anyway is not optimum in term of power savings.